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An Australian research team has found that some printers give off invisible particles as they operate, with the worst culprit emitting concentrations similar to those of secondhand tobacco smoke.

But that doesn't mean you need a mask every time you pass your printer.

The conclusions are based on a small subset of data, the researchers acknowledge, and raise many more questions than answers. Independent experts and a printer manufacturer say it would be premature to issue any warnings until researchers know exactly what chemicals make up the ultrafine particles. Some of the types of particles identified by the researchers can also be generated from simple activities such as burning a candle or making toast.

Even so, one of the researchers behind the study, physicist Lidia Morawska of Queensland University of Technology in Brisbane, Australia, said it wouldn't hurt to make sure printers are kept in open areas at a distance from users.

"This information should mainly guide manufacturers into designing printers that are low emitters," Morawska said. "In the meantime, if you can, put the printer in a separate well-ventilated area."

It's not clear that these ultrafine emissions are dangerous, according to Robert Hamers, the chairman of the chemistry department at the University of Wisconsin-Madison.

Each particle, up to 1,000 times smaller than a dust particle, is small enough that it can get drawn deeply into the lung's tiny sacs, Hamers said. But what it does once it gets there depends on its chemical composition.

For example, cigarette smoke is dangerous not because it contains tiny particles but because those particles include cyanide and carbon monoxide.

"Yes, printers generate particles, but probably much less chemically toxic than cigarette smoke," Hamers said. Until those particles can be analyzed, people ought not overreact, he added.

Morawska's research was conducted with Congrong He and Len Taplin and published Wednesday on the Web site of the journal Environmental Science & Technology.

The printer study came about accidentally, Morawska said. The research team was investigating how efficiently a building's ventilation system was filtering pollution from a nearby highway. But the investigators noticed unusually high particle measurements indoors.

The team methodically eliminated likely sources, including photocopy machines. Eventually it identified printers as the cause.

Using equipment that counts the number of ultrafine particles in a certain volume, the scientists studied all 62 printers in the building.

About 60 percent produced no detectable emissions. Of the others, about a quarter produced at least a 10-fold increase in the concentration of particles.

The emissions tended to be higher for printers using older cartridges and for those printing more ink-intensive pages. Nearly all printers were made by the industry leader, Hewlett-Packard Co.

Tuan Tran, the vice president of sales and marketing for HP's imaging division, said the company puts its printers through rigorous emissions testing. He said HP is trying to contact the researchers for clarification on their conclusions, some of which he said seemed odd.

For example, the Australian team found that one HP printer produced high emissions while a nearly identical model produced none.

"We want to make sure we have the facts before we jump to conclusions," Tran said. "We do quite a bit of work to make sure our products adhere to health standards around the world."

Charles J. Weschler, a chemistry professor at the University of Medicine and Dentistry of New Jersey, said people should worry about secondhand smoke or vehicle emissions, but not printers.

"It's an interesting study and it alerts us to the issue that it warrants further attention," he said, "but I would not change my behavior at this point based on a preliminary finding."